; s 10 =(a 1+a 10)* 10/2

100 =(a 1+ 100)* 10/2

a 1+ 100 = 100 * 2/ 10

a 1=-80

because

Sn=na 1+n(n- 1)/2*d

s 10 = 10 *(80)+ 10 * 9/2 * d

100=-800+45d

45d=900

d=20

General formula: sn =-80n+10n (n-1) =10n2-90n.